Field of the Invention
The present invention relates to a nuclear magnetic flowmeter. More specifically, the present invention relates to a nuclear magnetic flowmeter with a straight measuring tube through which a multiphase medium can flow. In embodiments, the nuclear magnetic flowmeter includes a magnetization device that is located around the measuring tube for producing a magnetic field in the medium flowing though the measuring tube. The nuclear magnetic flowmeter also includes at least one gradient coil for producing a gradient in the magnetic field and/or at least one signal coil that excites the medium, and/or detects a result of the excitation. Additionally, the nuclear magnetic flowmeter includes a first coil insulating frame and a housing. The housing has a first face side and a second face side along a longitudinal axis of the measuring tube. In the first face side, there is a first housing opening, and in the second face side there is a second housing opening through which the measuring tube is routed. The magnetization device is provided in the interior of the housing. The housing, aside from the two housing openings, is tightly sealed. The gradient coil and the signal coil are located in a space between the measuring tube and magnetization device, wherein the space is penetrated by the magnetic field. The at least one gradient coil and/or the signal coil or at least one signal coil are located on the first coil insulating frame.
Description of Related Art
The atomic nuclei of the elements which have a nuclear spin also have a magnetic moment which is caused by the nuclear spin. The nuclear spin can be construed as angular momentum which can be described by a vector. Accordingly, the magnetic moment can also be described by a vector which is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus in the presence of a macroscopic magnetic field is aligned parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of the precession is called the Larmor frequency ωL and is proportional to the value of the magnetic field strength B. The Larmor frequency is computed according to ωL=γB, in which γ is the gyromagnetic ratio which is maximum for hydrogen nuclei. The precession of the magnetic moment of an atomic nucleus is an alternating magnetic field with the Larmor frequency which can induce an electrical signal with the same frequency into a coil.
Nuclear magnetic resonance measurement methods are measurement methods that influence the precession of atomic nuclei of a medium in the presence of a macroscopic magnetic field through excitation by means of a controlled magnetic field and that evaluate a result of the excitation. The prerequisite for the measurement of a multiphase medium is that the individual phases of the medium can be excited to distinguishable nuclear magnetic resonances.
Nuclear magnetic flowmeters are measurement devices that implement nuclear magnetic resonance methods. They can measure flow velocities of the individual phases of the medium in a measuring tube and relative proportions of the individual phases in the multiphase medium. Nuclear magnetic flowmeters can be used, for example, for measuring the flow rate of a multiphase medium which has been conveyed from oil sources. This medium consists essentially of the liquid phases of crude oil and salt water, and the gaseous phase natural gas. All these phases contain hydrogen nuclei, which are necessary for nuclear magnetic resonances and are excitable to different nuclear magnetic resonances.
The measurement of the multiphase medium which has been conveyed from oil sources can also take place with test separators. The conveyed medium is introduced into a test separator over a time interval, wherein the test separator separates the individual phases of the medium from one another and determines the proportions of the individual phases in the medium. However test separators, in contrast to nuclear magnetic flowmeters, cannot reliably measure proportions of crude oil smaller than 5%. Since the proportion of crude oil of any oil source continuously decreases, and because the proportion of crude oil of a host of oil sources can already be less than 5%, it is not currently possible to economically exploit these oil sources using test separators. Further, in order to exploit such sources having very small proportions of oil, flowmeters that are accurate for mediums consisting of several phases are necessary. In particular nuclear magnetic flowmeters are suitable for this purpose.
In nuclear magnetic flowmeters, a magnetization device and a gradient coil produce a magnetic field along which the magnetic moments of the atomic nuclei of the multiphase medium are first aligned. The magnetic field produced by the magnetization device is homogeneous in the medium that is flowing through the measuring tube. The magnetic field is usually produced by permanent magnets that are located in the magnetization device, such as a Halbach array. Different measurement methods dictate a gradient in the magnetic field that penetrates the medium. This gradient is produced by superposition of the homogenous magnetic field with an inhomogeneous field produced by the gradient coil.
A controlled magnetic field that excites the precessing atomic nuclei can be produced by at least one signal coil. This signal coil, or other signal coils, can also be used as a sensor for an alternating magnetic field produced by the precessing atomic nuclei. Conventionally, the coils, (i.e. the gradient coils) and the signal coils are located in a space between the magnetization device and the measuring tube that is penetrated by the magnetic field.
In the interior of the nuclear magnetic flowmeter, a housing accommodates the magnetization device and tightly seals the interior relative to the exterior environment. The meaning of “tightly” depends on the purpose of the nuclear magnetic flowmeter. In embodiments, tightness can be specified for touching and foreign bodies. In other embodiments, tightness depends on moisture and water on the other according to “Degrees of protection provided by enclosures” (Standard EN 60529, published Jan. 1, 1992). Tightness for touching and foreign bodies is defined via the size of the foreign bodies and in the International Protection Code extends to dust-tightness (Ingress Protection Rating IP 6x). Tightness with respect to moisture and water can be given for example for temporary immersion (Ingress Protection Rating IPx7). The tightness can also be specified with respect to explosion protection, specifically by the type of explosion protection. Thus, the tightness can be specified by a pressure-tight encapsulation according to “Explosive atmospheres. Equipment protection by flameproof enclosures ‘d’” (Standard EN 60079-1, published Aug. 31, 2007) or by increased safety according to “Explosive atmospheres. Equipment protection by increased safety ‘e’” (Standard EN 60079-7, published Jan. 31, 2007).
Nuclear magnetic flowmeters conventionally have housings whose manufacture is complex due to the demands for tightness. In particular, the construction and production of the housing in the region between the measuring tube and the magnetization device is complex and expensive.